Most photographic film now used is 35 mm format film. Cameras have been developed to allow film of this format to be easily loaded by placing the film cassette, in which the film is stored, into an opening in the back of the camera and then shutting the back of the camera. As a free end or tongue of the film extends externally to the film cassette, this is used to automatically advance the film in the camera for picture taking. Once all the pictures have been taken, the film is rewound into the film cassette for removal from the camera for processing. In order for this to work, the free end or tongue of the film must extend externally to the cassette so that it can be pulled on to the take-up spool in the camera as the film is first loaded.
Photographic film tends to be processed in a single strip once the film has been removed from its cassette. Strips of negative film are processed by transporting them, either as a single individual strip or as a continuous length comprising two or more strips of shorter lengths, through a series of processing solutions in various tanks in the processing apparatus.
In known processing apparatus, the film strip is pulled through tanks containing the processing solutions either by a leader which is attached to the leading edge of the film strip, or by moving a rack or spiral containing the film strip from tank to tank. Individual film strips may be pre-spliced into a long reel with a leader card at the front end, clipped to a rack, or fed into a spiral.
Where the film strip is attached to a leader, it is unloaded from the cassette and attached to the leader in a manual operation. The leader is then fed into the processing apparatus so that the film can be processed as it is transported through the apparatus.
However, in some processing apparatus, there is a minimum length of film strip which can be processed. As a result, several film strips need to be spliced together prior to processing. This may be time-consuming as each strip will need to be measured to ensure that the minimum processing length is present in the spliced strip prior to processing.
Operations of splicing the film strips together or attaching the leader to the strip need to be carried out in darkroom conditions due to the sensitive nature of the film.
U.S. Pat. No. 4,411,725 describes daylight splicing apparatus in which film is guided through the apparatus under the control of a microprocessor based sensing circuit. Positional information of the film is fed to the sensing circuit via infrared positions sensors.
U.S. Pat. No. 4,947,685 discloses method and apparatus for measuring the repeat length of a moving web. The method includes forming detectable marks along the web at a repeated length interval. The length interval is subsequently detected by detecting one mark moving past a first detector and then detecting the same mark moving past a second detector which is spaced apart a given distance from the first detector, and measuring the time interval between the two readings. The repeat length is then determined using the separation of the detectors, the speed of the web and the time between detection of the mark by the two detectors.
U.S. Pat. No. 4,366,372 describes an arrangement for determining the length of a moving web by detecting a series of equidistant holes spaced along one edge of the moving web. The arrangement comprises an optical device for converting holes into light pulses which are then counted to give a measurement of the length of the web. Simultaneously, a mechanical encoder is used to provide pulses to a counter which counts down from a predetermined value. The difference in value between the mechanical counter and the optical counter provides an indication of deviation from a standard.
U.S. Pat. No. 4,906,854 discloses apparatus and method for the determination of positional information in relation to a film strip. The apparatus includes means for relating image area information with perforations arranged along the length of the film strip. The perforations along one edge of the film strip are counted by two spaced apart detectors which allow the desired information to be accessible even though one detector may not be able to read the perforations due to the presence of splice tape. An array is used to scan the image areas to determine the image area information.
However, in the apparatus described in U.S. Pat. No. 4,906,854, the two detectors for the perforations need to be spaced apart a distance which exceeds the length of any splicing tape which may be used to splice together two or more film strips. This apparatus has the disadvantage that short lengths of film strip cannot be readily measured.
It is therefore an object of the present invention to provide a film checking device which allows different lengths of film strip to be measured whilst still in a loading device for photographic processing apparatus, and for determining whether each strip exceeds the minimum length required which can be processed in the apparatus to which the loading device is attached.